testing a brake booster on a w123 - PeachParts Mercedes-Benz Forum

sleepstar · #1 06-20-2016, 02:27 PM

i'm testing a brake booster for suspected failure. with long and furious pumping on my mityvac i can get it to pull 2" of vac at which point if i stop pumping it bleeds off. i'm assuming this thing is supposed to hold steady vacuum but is 2" anywhere near the capacity? pedal is hard and requires pressure to stop the car but it feels like some assist is still there

ETA hose going into the booster pulls 24" of steady vacuum from the engine pump

one final edit: when the car is running , pressing the brake pedal you can hear a squeaky sound thats like air being let out of a balloon

junqueyardjim · #2 06-20-2016, 07:39 PM

Well you either have a leak in the vacuum booster or you might just have a leak in the seal between the master cylinder and the booster. It is very easy to replace that seal, inexpensive also. Mater of fact, I bought a couple extra and if you want one, i will cost you $10.00 shipped. It is not an "0" ring. The cross section of the "0" ring is round, the cross section of this seal is rectangular. I did put a new "0" ring in mine and it helped a little, but it is not the answer.

sleepstar · #3 06-20-2016, 07:44 PM

any way to tell the difference between the two? the MC was replaced maybe one or two years ago by a reputable shop so i'm assuming they replaced that seal as well...

BillGrissom · #4 06-20-2016, 07:53 PM

Quote:

Originally Posted by sleepstar

... i'm assuming they replaced that seal as well...

Some of those seals (square O-ring) w/ rebuilt MC's seem a bit thin to seal reliably. I have doubled-up on them or used a round O-ring from my kit.

But, w/ 24" Hg vacuum at the booster, sounds like your diaphragm inside is OK. Sometimes they get a crack that seals at different times. More commonly, a crack seals as the pedal is pushed, then you get sudden assist that throws you into the windshield.

It is normal to get atmospheric air flowing in by the pedal, since that is how the booster works. It flows thru the "air valve" as you move the pedal until the booster piston follows to close the valve. Normally, you should hear only a momentary hiss, if even that, so your air valve might be funky. In some boosters, you can buy just that part. A post here shows a booster disassembled.

I wouldn't judge by the Might-Mite test. I think most boosters slightly normally, and perhaps faster than you can hand pump, but again your air valve may not be sealing well or working perfectly.

sleepstar · #5 06-20-2016, 08:16 PM

i should clairfy, i get 24" from the vacuum pump on the engine down the booster hose, so its not a failing vacuum pump or a leaky booster hose. i was checking how well the booster held vacuum directly at the booster

and yeah its more than a slight hiss, it sounds really high pitched and squeaky, very obvious sound...

sleepstar · #6 06-21-2016, 02:26 PM

welp, tested a known good booster and i got it to hold 15" before i gave up pumping, so this one on my friends car is definitely gone south. any insight as to replacement? looks like four bolts on the inside and two at the booster where it attaches to the MC...?

BillGrissom · #7 06-22-2016, 03:37 PM

Yes, not hard to remove. Leave the MC bolted to the brake tubes and it will hang there fine and bend forward out of the way, and you won't have to open the hydraulics. Never priced a new one. If >$100, I would go junkyard though 30 yr old rubber isn't dependable. I only removed the booster on my 2 300D's to get the brake fluid out that leaked past a bad MC and to paint.